1. Field of the Invention
The invention relates to methods for producing nanoparticles, including the form of nanopowders, more particularly to methods for using shaped femtosecond laser pulses to produce nanoparticles and furthermore particularly to methods for using closed loop control of shaped femtosecond laser pulses to produce nanoparticles.
2. Brief Description of the Related Art
Nanoparticles are components which generally are small sized, less than 500 nanometers (nm), or more commonly less than 100 nm. Our invention allows for the precision control of nanoparticle formation under closed loop control using shaped ultrafast laser pulses, as a new tool, for materials discovery. Further our invention allows for the precision control of nanoparticle formation under closed loop adaptive control using shaped ultrafast laser pulses, as a new tool, for nanomaterials discovery.
Our invention provides for the ability to derive knowledge of nanoparticle formation as a function of laser pulse shape by controlling the rate (intensity) and type (coherent or incoherent) of energy deposition.
Our invention allows for the control of matter at the nanoscale in reaction regimes far from equilibrium. The invention provides an ability to synthesize and concomitantly characterize the nanomaterials produced while developing correlations relating laser pulse shape, energy deposition, and substrate or reactant composition to nanoparticle product size, size distribution and composition which are developed from the data obtained during its operation.
Our invention represents a new paradigm in the ability to elucidate the relationship between control of molecular reactions at the nanoscale and the properties of materials produced. The chemistry of the reacting system is integral to the control processor. The complexity of the chemistry and the characteristics of the products it yields depends on the sophistication of the instrumentation providing feedback about the desired relevant properties. The processor, that is, the closed-loop control engine, has inherent open architecture in that any desired property is integrated into the control process so long as the instrumentation exists or can be developed to provide information to the control processor at the relevant time scale.
The literature abounds with methods for the production of nanoparticles. Duval, 2000 and Heitmann, 2005 give several examples of nanoparticle production. Nanoparticle manufacture methods include precipitation from a high temperature melt, low solubility in aqueous environments, limited solution of powders into a matrix, precipitation in controlled volumes where the pore structure of a matrix dictates the solution domain from which a nanoparticle is grown, metal organic chemical vapor deposition (MOCVD), laser ablation, lithography, heterogeneous deposition on surfaces, low pressure (LP)CVD, plasma enhanced (PE)CVD, and deposition initiated by defect structures among other techniques. Some examples of these methods are described in Lowndes, 1998 and Li, 1998. The several journals devoted to the science are a testimony to the emergence of size as an independent degree of freedom and the tremendous potential impact that mastery of control at the nanoscale will have on all aspects of our world.
The development of a generalized tool for the production of high precision nanoparticles where particle size is invariant or at least less than a fraction of a nanometer may be impossible where the principal driving forces are dependent on statistical processes. Clever gas phase and liquid or solution phase processes have been developed but all suffer from a lack of generality. Processing conditions for one narrow particle size and composition, once found using conventional techniques, does not logically extend to the formation of other sizes often due to a range of complex interactions including encroachment of boundaries, fluid effects, etc. Solution processes, in particular, additionally suffer from impurity, stoichiometry and concentration effects which may confound the development of the desired properties of the nanoparticle even though the desired shape and size are achieved. Solvent impurities tend to concentrate on or within the nanoparticle precursor during precipitation. While the concentration of the impurity within the nanoparticle may be insufficient to dominate its crystallization and growth behavior they may poison or wash-out the desired size-dependent attributes.
One key to controlling particle formation at the nanoscale is a means to transfer the system from one state, representing a particular atomic or molecular array (the gas phase, for instance), into another state representing the desired array of atoms (a nano cluster with 1000 atoms, for instance). In a time-averaged processing method, characterized by, for example temperature, the system is changed through the evolution of enumerable possible states to a new average. The timescale for this evolution is long in comparison to the life-time in any one (micro)state from which the system may evolve. Since all (micro)states contribute to the new time evolved average, precise control through thermal means may be impossible and products at the nanoscale, that is, assemblies of particles, must reflect the dispersion of (micro)states from which they evolved and into which they will ultimately fall. If the rate at which energy input occurs is faster than the transition of the system from one (micro)state to another there exists the possibility of driving the transition to and from a particular single state (free from statistical variations) into another while avoiding the evolution across (through) enumerable undesired states.
Perhaps with this idea in mind, many researchers have explored the use of lasers to affect the rapid deposition of energy into a system to affect such transitions. Further the use of femtosecond lasers has been more recently explored because energy is placed into the system on a time scale which is short compared to the time scale of energy partitioning into nuclear modes and plasma modes.
Laser ablation involves the interaction of intense laser light with a solid target resulting in the formation of an energetic plume to affect formation of a multitude of structures. This discussion focuses on its application to the formation of nanoparticles. There have been numerous reports regarding the use of longer duration (100-500 fs) laser pulses to induce nanostructure and nanoparticle formation. These reports employ the ultrafast deposition of energy to induce a phase change on a time scale that is short for energy diffusion from the interaction region to the bulk and short on the time scale for plasma formation. While the present invention also employs ultrashort laser pulses, including ones having a pulse duration of less than 100 fs, our invention is further distinguished by the use of laser pulse shaping and the control of said pulse shapes by a control algorithm to affect the control of particle size and particle size distribution.
Lasers of various pulse lengths have been used in the laboratory and on the commercial scale to drive chemical vapor deposition, laser-assisted chemical vapor condensation, laser pyrolysis and other methods. A significant number of experiments by the inventors has demonstrated that simple variables like temperature, pressure and reaction zone geometry could be used to control the size of particular nanoparticles. For instance, film versus particle formation is largely a function of the pressure of the reactor. At low pressure collisions can occur primarily with the walls to produce films and at high pressure collisions can occur with other atoms to produce particles. Nanoparticles produced using such methods are amongst the most monodisperse, but are still far from being truly monodisperse. True precision at the nanoscale is not achieved, rather, the precursor laden liquid or gaseous substrate is excited by the laser followed by rapid condensation prior to particle growth. Particle monodispersity is achieved purely by geometrical constraints of the reaction zone caused by confinement of the region of intense heating to a localized region in space. Condensation product particles traveling outward have little opportunity to collide prior to loosing sufficient energy so that particle coalescence is improbable (for example, see Camata, 2000).
Femtosecond duration laser pulses have been used to drive crystallization of proteins from a saturated solution (see Masuhara, 2006; Adachi, 2004). In inverse laser ablation, a laser is used to drive dissociation, ionization dissociation and plasma formation from a gas of molecules, followed by coalescence of these species to yield solid nanoparticles. However, no particle size control was affected (Bililign, 1998). This method employed a ps laser at a single wavelength impinging on a super sonic jet of iron carbonyl to produce iron nanoparticles. The super sonic jet was used to eliminate post photochemical collisions. Their efforts included use of the 266-nm fourth harmonic at a energy of 2-4 mJ. Their work demonstrated that CO could be systematically removed from the metal carbonyl and that Fen clusters n=1, 2 . . . 7+ readily formed. Others used IR laser to irradiate trisilane in mixtures with “non-absorbing” carbon disulfide in the liquid phase. Their results showed that simultaneous decomposition of both compounds resulted in formation of Si/S/C/H polymer. They employed TEA CO2 laser operating at 944.19 cm-1 with a repetition frequency of 1 Hz and an energy of 1.2 J in a pulse to induce infrared multiphoton dissociation (see, Pola, 2008). None of the above methods employed the use of shaped laser pulses to affect or control particle size.
The area of shaped pulse interaction with solids is limited. One recent paper demonstrates that the amount of ion to neutral formation in Al desorption can be controlled using optical emission feedback. (see Guillermin, 2009). Work with dual fs laser pulses provided for the opportunity ablation from a two phase system. The first pulse caused melting and then the second pulse, which interacts with the liquid, strongly couples and leads to emission products unlike that of single shot work. (see Singha, 2008). That work did not employ shaped laser pulses nor did it derive control of size from the manipulation of the laser pulse shape. The nonlinear excitation possible with fs duration pulses has led to interesting observations in the processing of materials. In Englert, 2008, it was shown that nonlinear effects serve to create in solid substrates vaporization zones much smaller that the diffraction limited focal spot size when pulses have negative cubic chirp, but such use of shaped laser pulses was not employed to affect properties of the ejected mass, nor was the ejected mass characterized, nor was it known whether the ejected mass was composed of nanoparticles.
Our invention provides for nanoparticle formation control achieved through the use of a wide bandwidth extremely short shaped laser pulse. Work done by Levis (Levis, 2002) has shown that bond selective photochemistry of gas phase molecules is possible through powerful search algorithms and multi channel (256) control of the spectral composition of femtosecond laser pulses. In such experiments, the search algorithm directed the system to find the desired chemical event. The molecule and its fragments, in the case of time of flight mass spectroscopy, were a part of the computational process to find the desired solution. The particular laser pulse shape was referred to as a “photonic reagent” to convey the reaction concept: reactant molecule plus photonic reagent yields product molecule. This work did not teach the formation of nanoparticles from the molecular system. In our invention we employ a “substrate” and use a wide bandwidth fs shaped laser pulse to drive the substrate into a new state consisting of nanoparticles. We define a substrate as a solid, liquid or gas of simple or complex composition which when acted upon by a shaped fs laser pulse yields nanoparticles. Examples of a substrate of simple composition are single crystal or amorphous silicon surfaces, pre-activated molten silicon surface and silane. Many other types of substrates can be envisioned, and some examples are provided here. Substrates consisting of volatile metal compounds such as nickel carbonyl Ni(CO)4, cobalt triacetylacetonate Co(acac)3, or tungsten hexafluoride WF6 vapors yield metal nanoparticles of Ni, Co or W. Mixtures of metal carbonyls yield alloys corresponding to the composition of the said mixtures, for example Ni(CO)4 and Fe(CO)5 yield NiFe alloys. One embodiment of our invention allows for the shaped laser pulse excitation of an expanding gas jet to induce nanoparticle formation. Comparison of the control space to that of the resulting nanoparticle products serves as part of a comprehensive scheme to create knowledge. A natural consequence of our invention is the cataloging of laser pulse shapes and the nanopartide characteristics so produced. This then allowed for the production of nanoparticles of one variety or another through the selection of the appropriate photonic reagent and “substrate.”
With the development of advanced instrumentation and computational methods the invention has further utility in the control of the production of materials with specific secondary and tertiary properties including for example, optical, magnetic and electrical (transport properties) and catalytic activity, adsorption, hardness and strength, among many other properties. That is nanoparticle formation control can be guided by the other properties that are dependent on the properties of the nanoparticle provided the requisite assay or characterization tool that provides information about the desired characteristics or properties can be incorporated into the feedback loop.
The area of photonic reagent driven chemical and physical processes has been widely investigated over the past 10 years (see Levis, 1995). Laser pulse shaping has proven invaluable for a range of experiments focused on controlling processes from high harmonic generation to biomolecule dissociation in the gas phase, but work in these disciplines has not addressed nanoparticle formation. Lupulescu, 2004 described work where an intense femtosecond pulse has been shown to fragment the majority of bonds in a gas phase molecule, such as a cyclopentyl metal carbonyl through multiphoton excitation, but that work neither described nor eluded to the possibility of nanoparticle formation.
Advances in laser pulse shaping technology, slaved to pattern recognition learning control systems, have opened opportunities for studying the dynamics and chemical manipulation of virtually any system that can be introduced into a closed-loop control apparatus (see Levis, 2002). Using this approach it is possible to tailor the control laser to manipulate molecular dynamics to affect the desired chemistry. The requirement of optimizing a desired product requires the ability for introducing an adjustable control field having sufficiently rich structure. Construction of such a field is currently possible in the laboratory using the technique of spatial light modulation in conjunction with an ultrafast laser pulse. As demonstrated here, the method of closed loop control for laser-induced processes offers a way of selecting the appropriate pulse shape needed for nanoparticle formation control without having detailed knowledge of the relationship between laser pulse shape and nanoparticle properties.
In closed loop operations, the substrate, the laser pulse shaper, and a pattern recognizing learning algorithm form the elements for repeated cyclic operation to teach the laser how to control reactivity and affect the generation of one product over another. Once a laser pulse shape is found that affects the desired nanoparticle product, that pulse can be used repetitiously to produce that product. Our invention enjoin elements of the molecular reaction control process (Levis; 2002), namely closed-loop operations with strong field laser control, with new characterization techniques to the affect precision control at the nanoscale so as to produce nanoparticles of desired size, shape and ultimately more advanced properties.
A general schematic of the closed loop control for shaped laser pulse-induced nanopowder formation process is shown in FIG. 1. In this process loop, the phase and amplitudes of the component frequencies of a fs pulse are the control variables, and the resulting nanoparticle average particle size and particle size distribution as ascertained using, for example, a differential mobility analyzer, the number of particles having different masses are the observables employed to evaluate the fitness of the pulse shape. Using the fitness information derived from trying a variety of laser pulses, the control algorithm then creates new or modified pulse shapes to try based on the patterns recognized from the evolving measured pulse-shape fitness relationship. Successive turns through the control loop ultimately leads to the finding of a pulse shape that produces the desired nanoparticle properties.
The present invention differs from other laser driven processes in that shaped, fs, laser pulses are used to produce nanoparticles, and that different laser pulse shapes are capable of producing nanoparticle size distributions of different average size and size distribution.